1. Field of the Invention
The present invention relates to a method of preparing a polymer electrolyte composition and a method of manufacturing a lithium secondary battery using the same. In particular, the present invention relates to a method of preparing a polymer electrolyte composition and a method of manufacturing a lithium secondary battery using the same which has a high ionic conductivity, good mechanical properties, a stable interface characteristic, a good discharging characteristic at a high and low temperature and an efficient discharging characteristic.
2. Description of the Related Art
There has been a great deal of interest in developing better and more efficient methods for storing energy for applications such as cellular communication, satellites, portable computers and electric vehicles. In particular, great effort has been dedicated to the development of a lithium ion battery having a cathode including lithium, an anode including lithium or carbon and a non-aqueous electrolyte, because of its higher energy density than that of a lead storage battery or nickel-cadmium battery having an aqueous electrolyte.
Recently, the widely used lithium ion battery having a satisfactory ionic conductivity uses a liquid electrolyte, however, the leakage of the liquid electrolyte occurs frequently. Moreover, any leakage in the cell lessens the performance of the battery. Accordingly, lithium ion batteries are packed with an aluminum can and are provided with various protective devices, thereby enlarging the volume of the cell and reducing the energy density to an undesirable degree. Furthermore, such a lithium ion battery is not applicable to a battery having thickness of 3 mm or less.
In contrast, solid electrolytes are free from problems like leakage, however, these solid electrolytes tend to exhibit inferior properties compared to the liquid electrolytes. This is due to the fact that ionic conductivities for the solid electrolytes are often 5-100 times inferior to that of the liquid electrolytes.
In general, a polymer lithium secondary battery includes an anode, a polymer electrolyte and a cathode. The components are selected to satisfy various conditions of the secondary battery such as lifetime, capacity, temperature characteristic, stability, etc.
As for the components of the cathode applied to the secondary battery, lithium oxide complex (LiCoO2, LiMn2O4, LiNiO2) which has a laminated structure and lithium ion which can be inserted between layers or separated from layers can be used. As for the components of the anode, carbon compounds such as graphite compounds or coke can be used and these are examples of which include mesocarbon microbeads (MCMB) and mesophase carbon fiber (MPCF).
A polymer electrolyte which is widely used as a main component of the lithium battery is free from the leakage problem. The manufacturing of the battery using the polymer electrolyte does have some advantages however, the polymer electrolyte is required to have a good ionic conductivity, a thermal and electrochemical stability, a good mechanical strength and a good adhesiveness to the electrodes.
The polymer electrolytes which is currently used or which is under development include a main liquid-type organic solvent such as ethylene carbonate and propylene carbonate, a vice liquid-type organic solvent such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate, and polyvinylidene difluoride-based compounds (PVdF), polyacrylonitriles (PAN), polyethylene oxides, a copolymer thereof or a mixture thereof which can accept lithium salts such as LiPF6 and LiAsF6.
The polymer electrolyte including the polyvinylidene fluoride compounds has a good mechanical strength. However, the adhesiveness to the electrodes is not sufficient and this requires an adhering process using heat or pressure. The solvent might evaporate during the adhering process of the electrodes to the electrolyte. Thus, films which do not contain the electrolyte are adhered to the electrodes and then additional impregnation process is implemented to the solvent.
When polyacrylate polymer electrolytes having good affinity to the solvent are used in order to increase the adhesiveness to the electrodes, it can be accomplished, however, the mechanical strength of the electrolyte is not good.
One class of polymer electrolytes, especially, gel electrolytes in which liquid electrolyte is dispersed in a polymer matrix, includes a significant fraction of solvents in addition to the salt contained in the polymer matrix.
A method for preparing the gel electrolytes is disclosed in U.S. Pat. No. 5,456,000. A battery is assembled with a gel electrolyte to leave the electrolyte salt and solvent out of the polymer gel system until after the cell is completely fabricated. Thereafter, the solvent and the electrolyte salt may be introduced to the polymer system in order to swell the battery. This battery has an advantage of allowing the cell to be fabricated in a non-dry environment. However, a film is formed from a polymer containing a plasticizer in order to facilitate the impregnation of the polymer film with the solvent. As a result, the battery is assembled, the plasticizer is then extracted to form a micro-porous film and the solvent used for the extraction is evaporated. Such a process requires homogeneous impregnation of the polymer with the solvent and also requires many hours to lengthen the processing time.
In order to overcome the above-described problem, U.S. Pat. No. 5,219,679 discloses a method of preparing a polymer electrolyte after mixing the polymer and the liquid electrolyte. In this method, the solvent is already homogeneously dispersed into the polymer prior to the assembling of a battery. An additional process of extracting a plasticizer or drying is not necessary, however, the preparation of the polymer electrolyte and the assembling of the cell should be implemented under a dry condition. Furthermore, if the polymer electrolyte contains a large amount of solvent, the mechanical strength is poor. This makes a continuous processing being difficult and the electrical short being liable to generate.
U.S. Pat. Nos. 5,585,039, 5,639,573, 5,716,421 and 5,688,293 disclose polymer electrolytes prepared by filling polymer electrolytes into porous films which is good enough to overcome the problems of the mechanical strength. According to the method introduced in these patents, a filling process or a coating process of the electrolyte into or onto the porous film is additionally implemented, thus complicating the manufacturing process of the battery.
Therefore, it is preferred that a gel polymer electrolyte containing a polymer and a solvent is prepared, then an anode, a cathode and a polymer electrolyte obtained accordingly are integrated to manufacture the battery, which simplifies the manufacturing process of the battery.
In this case, since only one coating process is necessary for preparing the polymer electrolyte, the ionic conductivity, mechanical strength of the polymer and the solvent mixture and the interface adhesiveness to the electrodes are anticipated to exhibit good qualities. Since the polymer electrolyte impregnated with the solvent is integrated, a lamination method at a high temperature cannot be used. Accordingly, the polymer electrolyte should have good interface adhesiveness to the electrodes.
U.S. Pat. No. 5,849,433 discloses a method for preparing a polymer electrolyte using a material which has a good mechanical strength and adhesiveness in order to improve the mechanical properties. According to the method, the polymer electrolyte is prepared by forming a film from a mixture of materials having a good mechanical strength and adhesiveness to obtain a desired polymer electrolyte and by impregnating the film with a liquid electrolyte.
However, in this method, additional impregnation process of the polymer film with the liquid electrolyte is necessary to manufacture the polymer electrolyte.
It is an object of the present invention to provide a method of preparing a polymer electrolyte composition having a good mechanical strength and a good adhesiveness.
Another object of the present invention is to provide a method of manufacturing a lithium secondary battery which employs the polymer electrolyte composition having a good mechanical strength and a good adhesiveness.
To accomplish these objects, a method of preparing a polymer electrolyte composition is provided in the present invention. A polymer mixture including a) a polymer mixture which includes polyvinylidene fluoride-based polymer and b) at least one polymer selected from the group consisting of polyacrylonitrile and polymethyl methacrylate is mixed with a solvent in which a lithium salt is dissolved. The mixing ratio of the polymer mixture and the solvent is in the range of about 1:3-10. The obtained first mixture is stirred at a room temperature for about 1-48 hours. Then, the mixture obtained accordingly is heated at a temperature of about 60-250xc2x0 C. for about 5 minutes-6 hours while stirring.
Another object of the present invention is accomplished by the following method of manufacturing a lithium secondary battery. A polymer mixture including a) a polymer mixture which includes polyvinylidene fluoride-based polymer and b) at least one polymer selected from the group consisting of polyacrylonitrile and polymethyl methacrylate is mixed with a solvent in which a lithium salt is dissolved. A mixing ratio of the polymer mixture and the solvent is in the range of about 1:3-10. Thus obtained first mixture is stirred at a room temperature for about 1-48 hours. The mixture obtained accordingly is heated at a temperature of about 60-250xc2x0 C. for about 5 minutes-6 hours while stirring to prepare a polymer electrolyte composition. Then, the obtained polymer electrolyte composition is coated onto at least one substrate selected from a group consisting of a molded film, an anode and a cathode and then the composition is dried.
Polyvinylidene fluoride-based polymer includes a large amount of electrolyte and lithium salts and provides a good mechanical strength. Polymethyl methacrylate polymer has a good affinity to the solvent which adheres the electrolyte strongly to the electrodes. Polyacrylonitrile polymer has a good adhesiveness to the electrolyte, thus it improves the adhesiveness of the electrolyte to the electrodes without deteriorating the excellent mechanical properties of the polyvinylidene fluoride-based polymer.
According to the present invention, as a lithium secondary battery is manufactured by employing a polymer electrolyte, that is, prepared by an optimized method, a mechanical strength thereof is improved and an affinity of the electrolyte with the solvent can be improved to minimize the leakage and evaporation of the solvent in the polymer electrolyte. As a result, a lithium secondary battery having a stable charge/discharge characteristic and a high capacitance can be manufactured. Furthermore, since the electrolyte of the present invention has a good adhesiveness to the electrodes, a contacting resistance of thus obtained battery is small and a local concentration of a current can be prevented to improve the performance of the battery during charge/discharge thereof.